LTE (Long Term Evolution) is a long term evolution of the technology standards of the Universal Mobile Telecommunications System (UMTS) developed by the 3GPP organization. A LTE system has introduced some crucial transmission technologies, such as OFDM and the multi-antenna MIMO which significantly increase the spectrum efficiency and data transmission rate. The LTE system supports a variety of bandwidth allocations so that the spectrum allocation can be more flexible and the system capacity and coverage can be significantly improved. LTE wireless network tends to have a more flat architecture, which decreases the system latency and lowers the costs of network construction and maintenance. LTE R12 introduces Small Cell, i.e., a small cell base station which is a low-powered wireless access node and operates at licensed and unlicensed spectrum. Small Cell may be used indoors or outdoors, and it may cover a 10-meter indoor space or a 2 km range in the open field. In contrast, the coverage of a macrocell base station may be up to several kilometers. Small Cell integrates the technologies of femtocell, picocell, microcell, and distributed wireless. Small Cells are characterized by their intensive arrangements and overlapping coverage areas.
The wireless communication system sends and receives signals at the same frequency band, wherein if synchronization between cells fails to be maintained, there will be severe interference between the signal sending and receiving processes. TDD network, in particular, requires the cells to maintain precisely synchronized (in microsecond level) sub-frame boundaries and to be configured with the same uplink to downlink ratio in the same TDD synchronization area. Therefore, the synchronization and time service of a cell base station is a crucial matter in the wireless communication network. The Time service modes currently applied mainly include GPS synchronization, network synchronization, terminal measurement auxiliary synchronization, and air interface self-synchronization between base stations. The main idea of the air interface self-synchronization between base stations is that a base station can synchronize with other base stations in the network which have already been synchronized. Air interface self-synchronization includes initial establishment of synchronization and periodical maintaining of synchronization. In the phase of initial establishment of synchronization, a base station A, which is assumed to have synchronized with the absolute time, becomes the time reference for other base stations in the synchronization area for acquiring synchronization. The other based stations simulate the behavior of a terminal, and search for a synchronization signal from the base station A to adjust their respective timing to coincide with that of the base station A, so that synchronization among base stations can be obtained. In order to prevent clock drift, periodical tracking of synchronization signal is further required, which is a similar process as the initial establishment of synchronization. Time interval of the periodical tracking of a self-synchronization cell may be in terms of seconds or longer. The method of air interface self-synchronization between base stations requires the base stations to be able to monitor each other.
In the case of Small Cell, the base stations are more applicable in indoor situation, and GPS time service synchronization is severely restricted by the indoor penetration problem. Therefore, the communication between the base stations mainly relies on the air interface communication, air interface self-synchronization being the most commonly used method. However, because of the intensive arrangements of Small Cells, one small cell base station may monitor a plurality of neighboring base stations simultaneously, and thus there would be problems of time service confliction and time service selection. That is, if one Small Cell simultaneously monitors a plurality of neighboring Small Cells, said Small Cell needs to select one synchronization source from the plurality of neighboring Small Cells.